This invention relates to alkali metal detectors for detecting such metals in gas streams, and more particularly relates to the detection of alkali metals in combustion gas streams such as from pressurized fluidized beds.
In U.S. Pat. No. 4,282,741, issued Aug. 11, 1981 in the name of the present inventor, there is disclosed a device and method for detecting alkali metals. The teachings of this patent are directed to the use of such alkali metal detectors in coal gasification systems. In such systems the product gas from the gasifier is at a relatively low temprature, approximately 300.degree. F. However, these gases may contain alkali metals. These metals are generally considered to be undesirable contaminants whose presence is undesired because of the corrosive and other detrimental effects that these metals have in those cases in which the product gas is used in conjunction with a conventional gas turbine. Accordingly, it is desirable to have a device and method for detecting the presence of alkali metals in such product gas streams even if the alkali metal contaminants are present only in trace amounts.
Even though the device illustrated in the aforementioned patent to Zarchy is extremely useful in the detection of alkali metals in effluent gas streams from coal gasification systems, the apparatus disclosed therein does not function at the high temperature conditions encountered in pressurized fluidized bed systems. The pressurized fluidized bed combined cycle system is an advanced power generation system that offers many advantages over current technology. In addition to the advantage of using relatively abundant coal rather than oil, the pressurized fluidized bed cycle power plant exhibits lower installation and lower operating costs than many generating plant designs. Additionally, this overall systems exhibits the ability to meet or exceed current air quality standards. It is known, though, that the effluent from a pressurized fluidized bed combustor contains undesirable levels of particulate matter containing high levels of alkali compounds. Fairly conventional techniques may be used to measure and quantify the particulate content of the effluent stream from such beds. Hence, with respect to this process contaminant, it is relatively easy to quantify, measure and evaluate various product gas cleanup systems. However, prior to the development of the instant apparatus, quantification of alkali metal vapor concentrations in such systems were not possible to obtain. A significant reason for the lack of this quantification ability is the significantly high temperature in the product gas from a pressurized fluidized bed. Such gases typically exhibit a temperature between approximately 1,800.degree. F. and approximately 2,000.degree. F. At such temperatures, the only practical material for providing electrical insulation for electrical feed-through connections are ceramic materials. However, even these materials, at these temperatures, exhibit conductivities which are sufficiently high to prevent accurate readings. It is recalled from the aforementioned patent to the instant inventor, which is hereby incorporated herein by reference, that the current levels of interest lie in the nanoampere range. At these extremely low current levels, even ceramic feed-through insulating materials are, without more, incapable of providing the high degree of electrical isolation required. The low levels of current found in such detector systems are a direct consequence of the fact that alkali metal concentrations extending all the way down to accuracies of a part per billion are required for proper monitoring and control of alkali metal concentrations in the product gas from the fluidized bed. It should furthermore be pointed out that in the effluent gas from pressurized fluidized beds the alkali metals are present mostly in the vapor phase rather than in the particulate form. This is a direct consequence of the high temperatures involved and does provided one advantage in that it permits in situ measurement of the alkali metal concentration in the present invention. Accordingly, since the detector can be mounted in situ, the readings are uncomplicated by sampling errors.